Industrial processes used for the manufacture of a wide variety of valuable goods are often automated such that the processing equipment performs without direct control of human operators. Sensors are typically utilized in such situations to measure aspects of the processing operation, either for direct feedback control, or for confirmation that processing conditions are within established norms.
Plasma is used in various types of industrial processes in the semiconductor and printed wiring board industries, as well as in various other industries such as in the medical equipment and automotive industries. One common use of plasma is for etching away materials in an isolated or controlled environment. Various types of materials may be etched by one or more plasma compositions, including glasses, silicon or other substrate materials, organics such as photoresist, waxes, plastics, rubbers, biological agents, and vegetable matter, and metals such as copper, aluminum, titanium, tungsten, and gold. Plasma is also utilized for depositing materials such as organics and metals onto an appropriate surface by various techniques, such as via chemical vapor deposition. Sputtering operations may also utilize plasmas to generate ions which sputter away material from a source (e.g., metals, organics) and deposit these materials onto a target such as a substrate. Surface modification operations also use plasmas, including operations such as surface cleaning, surface activation, surface passivation, surface roughening, surface smoothing, micromachining, hardening, and patterning.
Plasma processing operations can have a significant effect on a company's profit margin. This is particularly true in the semiconductor and printed wiring board industries. Consider that a single semiconductor fabrication facility may have up to 200-300 processing chambers and that each processing chamber in commercial production may process at least about 15-20 wafers per hour. Further consider that an eight inch wafer which is processed in one of these chambers in some cases may be used to produce up to 600 individual semiconductor chips which are each worth at least about $125, and that each of these semiconductor chips are in effect “pre-sold.” Therefore, a single wafer which has undergone an abnormal plasma process and which is scrapped can result in lost revenues of at least about $75,000.
The particular plasma process which acts on the wafer such that a semiconductor device may be formed therefrom is commonly referred to as a plasma recipe. Plasma processes may be run on wafers in a commercial production facility in the following manner. A cassette or boat which stores a plurality of wafers (e.g., 25) is provided to a location which may be accessed by a wafer handling system associated with one or more processing chambers. One wafer at a time is processed in the chamber, although some chambers may accommodate more than one wafer at a time for simultaneous plasma processing. One or more qualification wafers may be included in each cassette, and the rest are commonly referred to as production wafers. Both the qualification and production wafers are exposed to the same plasma process in the chamber. However, no semi-conductor devices are formed from a qualification wafer as qualification wafers are processed and retained solely for testing/evaluating the plasma process, whereas semiconductor devices are formed from the production wafers. Further processing operations of these now plasma processed production wafers are required before semiconductor devices are actually formed from such production wafers.
Monitoring is employed in connection with many plasma processes to evaluate one or more aspects of the process. One common monitoring technique associated with plasma recipes run on wafers is endpoint detection. Endpoint detection is concerned with identifying when one or more steps of a given plasma recipe is/are complete, or more specifically that point in time when the predetermined result associated with a plasma step has been produced on the product (e.g., when a layer of a multi-layered wafer has been completely removed in a manner defined by a mask or the like). Many endpoint detection techniques operate by identifying the point in time when the intensities of particular wavelengths or spectral bands of optical energy emitted from the plasma processing chamber change. Such intensity changes result, for example, from a layer being completely etched away and material from a lower, different layer beginning to be removed and dispersed within the chamber, as well as, for example, various gases used in the plasma process no longer being consumed at the same rate when the layer is substantially removed.
As such, typical endpoint detection techniques are not concerned with identifying abnormal conditions that may occur during the processing of a particular wafer nor evaluating trends that may be occurring within a processing chamber over time as multiple wafers are processed in accordance with a particular plasma recipe. Commonly used endpoint detection techniques provide no information on how the plasma process has actually proceeded or the “health” of the plasma process—only if and when an endpoint of the subject plasma step has been reached. Other monitoring techniques that are commonly used in plasma processes suffer from this same type of deficiency. Pressures, temperatures, and flow rates of the feed gases used to form the plasma are commonly monitored. Various aspects relating to the electrical system associated with the plasma are also monitored, such as the power settings being utilized since this will affect the behavior of the plasma. However, these types of monitoring operations do not necessarily provide an indication of how the plasma process is actually proceeding. All of the “hardware” settings may be correct, but still the plasma may not be performing properly for a variety of reasons (e.g., an “unhealthy” plasma). Since errors in a plasma process are typically detected by some type of post processing, destructive testing technique, multiple wafers are typically exposed to the faulty plasma process before the error is actually identified and remedied resulting in many wafers that need to be scrapped at great cost.
In addition to endpoint detection techniques, various techniques are known for monitoring the “health” of a plasma process as it is performed on a group of wafers. One such plasma health monitoring technique relies on pattern recognition techniques to determine if optical spectra from the processing chamber match at least one previously stored “normal” or “healthy” spectrum. As may be appreciated, such techniques can be quite computationally intensive due to the amount of spectral data involved and also require the establishment of normal spectra which may be searched for a match. In one known plasma process monitoring technique, principal component analysis (PCA) has been used to reduce the amount of spectral data that must be processed to a more manageable size.